Cryptographic schemes for a local cable company

This, for sure, is one of my favorite questions during my university year.

Q: A local cable company is improving the security of their method for distribution of “pay per view” movies. Each subscriber is given a secret key Ki at subscription time; this is programmed into that users black box on top of their TV, and kept in the company’s secured database. A movie M, which consists of a string of many gigabits
of data, is delivered to a subscriber’s home by cable. A user’s request for a movie is authenticated by a data value which is a function of Ki, sent from the access box to the cable company. Ki is also used to recover a special “movie key” as described below.

• Scheme I. The movie M is encrypted under Ki for each user Ui that requested to view
the movie, and such user Ui is sent the movie as a different string of bits EKi(M).

Setting up a PBX in your house (part 1)

In these series, I will talk about how to set up a PBX in your house. We will use a free, open-source PBX called Asterisk. But wait a minute! What the heck is PBX? Asterisk… what is this?

Since I will cover a lot of ground, I will break these series down into three parts:

Part 1: What is a PBX? Why should you care?

Part 2: What is Asterisk? And how to set it up?

Part 3: Asterisk Basic Hacks